Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create a beam of charged ions, which is then directed toward the wafer. As the ions strike the wafer, they impart a charge in the area of impact. This charge allows that particular region of the wafer to be properly “doped”. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these wafers can be transformed into complex circuits.
A block diagram of a representative ion implanter 100 is shown in FIG. 1. Power supply 101 supplies the required energy to the ion source 102 to enable the generation of ions. An ion source 102 generates ions of a desired species. In some embodiments, these species are mono-atoms, which are best suited for high-energy implant applications. In other embodiments, these species are molecules, which are better suited for low-energy implant applications. The ion source 102 has an aperture through which ions can pass. These ions are attracted to and through the aperture by electrodes 104. These exiting ions are formed into a beam 95, which then passes through a mass analyzer 106. The mass analyzer, having a resolving aperture, is used to remove unwanted components from the ion beam, resulting in an ion beam having the desired energy and mass characteristics passing through resolving aperture. Ions of the desired species then pass through a deceleration stage 108, which may include one or more electrodes. The output of the deceleration stage is a diverging ion beam.
A corrector magnet 110 is adapted to deflect the divergent ion beam into a set of beamlets having substantially parallel trajectories. Preferably, the corrector magnet 110 comprises a magnet coil and magnetic pole pieces that are spaced apart to form a gap, through which the ion beamlets pass. The coil is energized so as to create a magnetic field within the gap, which deflects the ion beamlets in accordance with the strength and direction of the applied magnetic field. The magnetic field is adjusted by varying the current through the magnet coil. Alternatively, other structures, such as parallelizing lenses, can also be utilized to perform this function.
Following the angle corrector 110, the ribbon beam is targeted toward the workpiece. In some embodiments, a second deceleration stage 112 may be added. The workpiece is attached to a workpiece support 114. The workpiece support 114 provides a variety of degrees of movement for various implant applications.
Referring to FIG. 2, a traditional ion source that may be incorporated into the ion implanter 100 is shown. The ion source 102 may include a chamber housing 10 that defines an ion source chamber 14. One side of the chamber housing 10 has an extraction aperture 12 through which the ions pass. In some embodiments, this aperture is a hole, while in other applications, such as high current implantation, this aperture is a slot.
A cathode 20 is located on one end of the ion source chamber 14. A filament 30 is positioned in close proximity to the cathode 20, outside of the ion chamber. A repeller 60 is located on the opposite end of the ion source chamber 14.
The filament 30 is energized by filament supply voltage 54. The current passing through the filament 30 heats it sufficiently (i.e. above 2000° C.) so as to produce thermo-electrons. A bias supply voltage 52 is used to bias the cathode 20 at a substantially more positive voltage than the filament 30. The effect of this large difference in voltage is to cause the thermo-electrons emitted from the filament to be accelerated toward the cathode. As these electrons bombard the cathode, the cathode heats significantly, often to temperatures over 2000° C. The cathode, which is referred to as an indirectly heated cathode (IHC), then emits thermo-electrons into the ion source chamber 14.
The arc supply 50 is used to bias the ion chamber housing 10 positively as compared to the cathode. The arc supply typically biases the housing 10 to a voltage about 50-100 Volts more positive than the cathode 20. This difference in voltage causes the electrons emitted from the cathode 20 to be accelerated toward the housing 10.
A magnetic field is preferably created in the direction 62, typically by using magnetic poles 86 located outside the chamber. The effect of the magnetic field is to confine the emitted electrons within magnetic field lines. The emitted electrons, electro-statically confined between cathode and repeller, take the spiral motions along the source magnetic field lines, thus effectively ionize background gases, forming ions (as shown in FIG. 3).
Vapor or gas source 40 is used to provide atoms or molecules into the ion source chamber 14. The molecules can be of a variety of species, including but not limited to inert gases (such as argon or hydrogen), oxygen-containing gases (such as oxygen and carbon dioxide), nitrogen containing gases (such as nitrogen or nitrogen triflouride), and other dopant-containing gases (such as diborane, boron tri-fluoride, or arsenic penta-fluoride). These background gasses are ionized by electron impact, thus forming plasma 80.
At the far end of the chamber 14, opposite the cathode 20, a repeller 60 is preferably biased to the same voltage as the cathode 20. This causes the emitted electrons to be electro-statically confined between cathode 20 and repeller 60. The use of these structures at each end of the ion source chamber 14 maximizes the interaction of the emitted electrons with the background gas, thus generating high-density plasmas.
FIG. 3 shows a different view of the ion source of FIG. 2. The source magnet 86 creates a magnetic field 62 across the ion chamber. The cathode 20 and repeller 60 are maintained at the same potential, so as to effectively confine the electrons, which collide with the background gas thus generate the plasma 80. The electrode set 90 is biased so as to attract the ions to and through the extraction aperture 12. These extracted ions are then formed into an ion beam 95 and are used as described above.
The above described technique of generating ions is highly effective for high-energy implant applications. Applications using high implant energies typically utilize mono-atoms, which are preferably created through the use of emitted electrons via an indirectly heated cathode. The indirectly heated cathode coupled with the magnetic fields, creates an environment where molecules are broken down into mono-atomic ion species. In these applications, source gas which breakdown into mono-atoms, such as H2, NF3, and B2H6, are supplied to the ion chamber. However, there are applications where such ions are not desirable. For example, there are applications that require ultra shallow junction formation, obtained with very low energy implants. Due to their inefficiency of beam transport, low energy applications preferably require the use of heavier charged molecules. These heavier molecules, such as decaborane, carborane and others, cannot be ionized using the above technique, since the high temperature environment would break apart the heavy molecules into smaller molecules or atoms. It is important for these applications that the molecules retain their molecular structure, losing only electrons before being extracted from the chamber.
Therefore, to create these heavier ions, alternative ion sources are typically used. In most cases, the ion source operates at much lower temperatures to preserve the molecular structure of the target species. In some embodiments, RF power is used to ionize the molecules.
Thus, there are two distinct modes of operation; one used for generating atomic ion species for high-energy applications, also known as hot mode, and a second for generating molecular ion species for low-energy applications, also known as cold mode. Because there are two distinct modes, there are typically separate ion sources, depending on the application and the source molecules. This complicates the ion implanter, and increases cost and complexity. A single ion source that can effectively generate ions for use in both modes, i.e. mono-atomic ions for high-energy implant applications and molecular ions for lower-energy implant applications, would be very beneficial.